System and method for sorbtion distillation

ABSTRACT

A system for distilling water is disclosed. The system comprises a heat source, and a plurality of open-cycle adsorption stages, each stage comprising a plurality of beds and an evaporator and a condenser between a first bed and a second bed, wherein each bed comprises at least two vapor valves, a plurality of hollow tubes, a plurality of channels adapted for transferring water vapor to and from at least one of the condenser or the evaporator, a thermally conductive water vapor adsorbent, and wherein each vapor valve connects a bed to either the condenser or the evaporator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation U.S. application Ser. No. 16/590,758,filed Oct. 2, 2019 which is a continuation of U.S. application Ser. No.16/164,942, filed Oct. 19, 2018, which is a divisional of U.S. patentapplication Ser. No. 15/637,236, filed on Jun. 29, 2017. Thisapplication also claims benefit of U.S. Patent Application No.62/356,126, filed Jun. 29, 2016, which is hereby incorporated in itsentirety by reference.

BACKGROUND

Population growth, increasing precipitation variability from climatechange, and aquifer depletion will result in water stress for over halfthe world population, >5 billion people, by 2050 (see C. A. Schlosseret. al., “The Future of Global Water Stress: An Integrated Assessment,”MIT, Cambridge, Mass., MIT Joint Program on the Science and Policy ofGlobal Change 254, 2014.). Desalination capacity is growing globally andwithin the US as water usage exceeds natural capacities. Grid-poweredreverse osmosis (RO) is currently the most favored technology, butrequires electricity, which remains mostly fossil-based.

Many review papers have been published comparing conventional andadvanced desalination (see O. K. Buros, The ABCs of Desalting.:International Desalination Association, 2000; O. A. Hamed, “Overview ofhybrid desalination systems—current status and future prospects,” SalineWater Conversion Corporation (SWCC), Al-Jubail, Saudi Arabia, 2004; M.T. Ali et. al., “A comprehensive techno-economical review of indirectsolar desalination,”; Renewable and Sustainable Energy Reviews, vol. 15,pp. 4187-4199, 2011; J. E. Miller, “Review of Water Resources anddesalination technologies,” Sandia National Laboratories, Albuquerque,N. Mex., SAND Report 2003-0800, 2003; S. Chaudhry. (2012, October) Newand Emerging Desalination.http://www.iapws.org/minutes/2012/Symp-Chaudhry.pdf; J. Tonner,“Barriers to thermal desalination in the United States,” U.S. Departmentof the Interior Bureau of Reclamation, Denver, Colo., Desalination andWater Purification Research and Development Program Report 144, 2008.)

Miller's 2003 SAND report succinctly describes the challenges of thermalprocesses: “All thermal distillation processes have one notable AchillesHeel, and that is the large amount of energy it takes to evaporate water(about 2200 kJ/kg) compared to the theoretical minimum energy requiredfor desalination (3-7 kJ/kg)”. Mechanical energy is easier to reuse,therefore reverse osmosis has become the most competitive desalinationtechnique. The largest desalination plant being built in the US, the SanDiego Carlsbad plant [Carlsbad Desalination Project, “Energyminimization and greenhouse gas reduction plan,” San Diego, Calif.,2008], uses RO and achieves an estimated energy intensity of 3.6kWh_(e)/m³ (13 kJ_(e)/kg) after upgrades to state-of-the-art pressureexchangers.

Conventional thermal desalination techniques such as multiple-effectdistillation (MED) and multi-stage flash (MSF) plants have been limitedto gained output ratio (GOR/PR) of around 10 for several decades. Thegained output ratio (GOR) is the ratio of input steam mass to productwater mass. It is equivalent to the performance ratio (PR) which is kgof product water per 2326 kJ or lbs. of product water per 1000 BTUs.Simple single stage distillation would have a GOR or PR of 1.Improvements to the efficiency of these pure thermal cycles have comefrom using higher exergy energy to recycle low temperature latent heats.High pressure steam drives thermal vapor compression (TVC) andmechanical energy is used in mechanical vapor compression (MVC).However, these techniques incorporate power generation equipment toconvert thermal energy to higher exergy input. Desalination usingelectrical or mechanical energy can seem more efficient as theyoutsource thermal losses to the energy conversion process. For example,Dean Kamen's Slingshot is a MVC distiller with an energy intensity of 24kWh_(e)/m³, but generates electricity using a 15% efficient Sterlingengine (see S. L. Nasr. Howstuffworks.http://science.howstuffworks.com/environmental/green-tech/remediation/slingshot-water-purifier2.htm).

Solar thermal desalination faces challenges on two fronts: reducingenergy intensity and collecting solar energy cost effectively. Therewould be immense benefit if direct solar-powered desalination could bemade cost-competitive with grid-powered reverse osmosis.

A rapidly deploying, portable, and dynamically sized desalinator cansignificantly reduce the risk of stranded cost and barriers to entry. At16,000 gallons per day (gpd) for each unit, a 1 Mgpd plant composed of63 units could be transported across the US by a single train. Comparedto current long lead-time desalination plants, time to water productioncould be reduced from a decade to weeks.

SUMMARY OF THE INVENTION

Disclosed is a distillation system, comprising a heat source and aplurality of open-cycle adsorption stages, each stage comprising aplurality of beds; and an evaporator and a condenser between a firststage hot adsorbent bed and a first stage cold adsorbent bed. In thisembodiment, each bed comprises at least two vapor valves switching vaporflow between each bed and either the condenser or evaporator of the samestage, a plurality of hollow tubes, a plurality of channels adapted tofacilitate water vapor flow between either the condenser or theevaporator and the bulk of either of the adsorbent beds. Each adsorbentbed is composed of a porous media, a hygroscopic material, and aplurality of graphite flakes.

Also disclosed is a method for distilling water. This method utilizes aplurality of stages, each stage comprising a hot adsorbent bed and acold adsorbent bed, and functions by repeating cycles of a forcing phasefollowed by a relaxing phase. The forcing phase comprises the steps ofproviding a heat source to heat the hot bed of a first stage to a firsttemperature, desorbing water vapor from the hot bed of the first stageand flowing the water vapor into a first condenser, condensing watervapor in the first condenser to form a liquid water and removing atleast some of the liquid water from the first condenser, providing asolution comprising water and at least one dissolved impurity to a firstevaporator, transferring the latent heat from the first condenser to thefirst evaporator to partially evaporate the solution comprising waterand at least one dissolved impurity to form water vapor and providingthe remaining more concentrated solution to an evaporator of asubsequent stage, adsorbing water vapor from the first evaporator intothe cold bed of the first stage, and transferring the heat of adsorptiongenerated by the cold bed of the first stage to heat a hot bed of asecond stage to a second temperature less than the first temperature.These steps are repeated for each of the plurality of stages until eachof the beds has had water vapor desorbed from the bed or adsorbed intothe bed. The relaxing phase comprises the steps of transferring bothsensible heat and latent heat of adsorption from the hot bed of thefirst stage to the cold bed of the first stage. As the hot bed of thefirst stage reduces in temperature, it adsorbs water vapor from theevaporator of the first stage, while the increase in temperature of thecold bed in the first stage causes it to desorb water vapor into thefirst stage condenser, condensing water vapor to form a liquid water andremoving at least some of the liquid water from the first stagecondenser. A solution comprising water and at least one dissolvedimpurity is provided to the first stage evaporator, transferring thelatent heat of vaporization from the first stage condenser to the firststage evaporator to evaporate said solution forming water vapor andproviding the remaining more concentrated solution to the next stageevaporator, with the water vapor from the first stage evaporator flowinginto and being adsorbed by the hot bed of the first stage. These stepsare repeated for each of the plurality of stages. During this relaxationstage, a reduced amount or zero amount of heat from an external sourceis needed to drive the distillation process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 show embodiments of a distiller.

FIG. 5 depicts an exploded view of one embodiment of an adsorption bed,excluding the coil manifold.

FIG. 6 depicts a more detailed view of an embodiment of an adsorbent bedcoil.

FIG. 7 is a drawing of a single adsorption stage with sectional cutoutson one bed to highlight internal structures.

FIG. 8 depicts an embodiment of a single stage of a distiller.

FIG. 9 shows a representative uptake diagram.

FIG. 10 depicts an embodiment of a system employing the distiller.

DETAILED DESCRIPTION OF THE INVENTION

The disclosed adsorption distiller uses an adsorption bed to reduce thewater vapor partial pressure above the evaporator, making thevaporization of water more efficient by reducing the required thermaldrive. By recycling both the latent energy of vaporization and the heatof adsorption 32 times from the heat source to the heat exhaust, thiscycle can achieve a Performance Ratio of 28, or 23 kWh thermal energy+0.1 kWh electrical energy per cubic meter of distilled water, whenlosses are included. This is about 3 times more efficient than existingthermal distillation techniques such as Multi-Stage Flash (MSF),Multiple Effect Distillation (MED), and at least 30 times more efficientthan single-effect solar stills. Due to its simple design and the use ofcommodity adsorbent materials, a bill of a materials analysis estimatesa reasonable cost for a 60 m³/day solar-powered distiller which includessolar collectors and gravity-driven sand pretreatment. Because thedistiller does not consume electricity or membranes, is highlyautomated, and requires modest pretreatment, the breakeven cost of wateris relatively low without financing costs, even with moderate interestrates, assuming a 25 year distiller life.

The adsorption distiller uses an inexpensive industrial nanomaterial,silica gel, as a highly porous matrix for a hygroscopic salt, calciumchloride. This composite of hygroscopic calcium chloride impregnated inthe internal pore surfaces of mesoporous silica gel has been studiedsince its discovery in 1996 by Aristov who called it a Selective WaterAdsorbent (SWS). Note that there can be some ambiguity in theterminology used to describe the reaction, because while chemicalabsorption is occurring, reaction kinetics is enhanced by using anadsorbent to increase surface area and vapor transport. As describedherein, the words “adsorption” and “adsorbed” are used to describe thereaction. Mesoporous silica gel has an average pore diameter of 15 nmwith surface areas of, in many cases, about 400 m²/gram but isrelatively inexpensive. By confining a salt within the silica gel pores,SWS boosts uptake (adsorbed water mass per mass of adsorbent) above thephysical adsorption capacity of silica gel. SWS also maintains a solidstate with a very large reaction surface area. In the envisioned design,a single 16 stage distiller packaged in a shipping container would havea vapor adsorption area of 4160 square kilometers, more than the area ofRhode Island.

The adsorption distiller consists of a number of open-cycle adsorptionstages connected in series, where the exhaust heat from an upper stageis used to drive the next stage. In a thermally driven heat pump, heatfrom a hot source is used to move heat from a cold evaporator to awarmer condenser. In the adsorption distiller, the evaporator andcondenser are kept nearly isothermal using a high heat transfercoefficient flat plate condenser/evaporator. Since the source of watervapor is from the input liquid being distilled, this configurationmaximizes the number of adsorption/desorption steps for any giventemperature gradient. In the adsorption distiller, two features lead tothe improvement in performance ratio. First, a large number of stagesare chained serially. Second, the adsorption beds are arranged in such away that one half of the cycle thermally drives a pair of adsorptionbeds out of equilibrium, while the other half is a relaxation towardsequilibrium that requires no energy input. Since both halves of thecycle generate distillate, the theoretically efficiency is equal to thenumber of beds, or double the number of stages since each stage has apair of adsorption beds.

Adsorption heat pumps based on silica gel have been studied for manyyears. However, one of the major difficulties with any silica gel basedsystem has been inefficient heat transfer due to the low thermalconductivity of silica gel. The heat of adsorption has a value within5-10% of the heat of vaporization, and can quickly raise the temperatureof the adsorbent and slow or stop the adsorption process if noteffectively removed. Previous attempts have used clay binders,waterglass, and conductive epoxies to thermally couple the silica gelwith expensive extended metal heat sink structures. This issue isparticularly important for the adsorption distiller as it relies onsmall temperature differentials, so the adsorbent temperature cannotelevate significantly during adsorption. We have based our design on apromising solution.

One approach uses expanded graphite, which can be thought of as agraphene precursor, where graphite particles have been sheared apart toa low number of carbon planes. When mixed with silica gel andmechanically compressed, the planar graphite particles align into sheetsand dramatically improve inter-particle thermal transport in the planeperpendicular to the compression direction, increasing the in-planethermal conductivity to 19 W/(m·K), a several hundred-fold improvement.The improvement in thermal conductivity and the recent availability ofindustrial quantities of expanded graphite and graphene precursors,allow us to design a greatly simplified adsorption bed using an array ofvertical tubes to form a closed-loop boiler that conveys the heat ofadsorption between stages using water vapor.

While flakes having a many layers are envisioned, the graphite flakespreferably have 100 layers of carbon planes or less. One embodimentcomprises flakes having 100 layers of carbon planes in each flake, or aflake thickness of about 0.034 micron. Another embodiment comprisesflakes having 50 layers of carbon planes in each flake. Anotherembodiment comprises flakes having 25 layers of carbon planes in eachflake. Another embodiment comprises flakes having 10 layers of carbonplanes in each flake. And yet another embodiment comprises flakes having1 layer of carbon in each flake.

Additionally, while flakes may be of any dimensions, the graphite flakesare preferably below 300 microns in size (roughly 48 mesh or larger).One embodiment comprises flakes between 180 and 300 microns in size(approximately 48 to 80 mesh). Another embodiment comprises flakesbetween 150 and 180 microns in size. Another embodiment comprises flakesbetween 75 and 150 microns. And another embodiment comprises flakes lessthan 75 microns in size.

Additionally, while any concentration of graphite is envisioned for thegraphite-salt composition, compositions comprising 50% or less graphiteby weight are preferred. One preferred embodiment comprises between15-30% graphite by weight. In one embodiment, the composition is binary,with the salt in silica gel making up the remainder of the weight.However, in other embodiments, the composition also includes additionalmaterials, including but not limited to biologics, polymers orcatalysts.

Cycle Operation

The disclosed system's cyclical operation is shown schematically inFIGS. 1 and 2 to illustrate the desalination cycle in both phases. Inboth figures, vapor flows are represented as dotted lines (see, e.g.,120, 122), heat flows are shown as dashed lines (see, e.g., 121, 123),brine flows as solid lines (see, e.g., 61, 130) and product water flowsas long dashed lines (see, e.g., 62).

Each bed has an upper and lower temperature limit, where there ispreferably less than about 20° C. difference between the upper and lowerlimit, and more preferably less than about 10° C. difference. Thehighest upper temperature being in the first hot chamber (21), whichpreferably has a temperature range of about 105 to 210° C., and morepreferably from 143.5 to 150.0° C. As will be seen, the lowertemperature limit of one chamber is the upper temperature limit of thenext chamber. In this figure, the first hot chamber (21) is connectedwith the first cool chamber (22), and the first cool chamber (22)preferably has a temperature range of about 138.1 to 143.5° C., or anarrow range (typically less than about 6° C.) below that of the firstchamber. The next chamber is the second hot chamber (23) whichpreferably has a temperature range of about 133.5 to 138.1° C., or anarrow range (typically less than about 6° C.) below that of the firstcold chamber. The second cool chamber (24) preferably has a temperaturerange of about 129.5 to 133.5° C., or a narrow range (typically lessthan about 5° C.) below that of the second hot chamber. The thirdchambers (28) and (29) have preferred temperature ranges of about 125.6to 129.5° C. and about 122.2 to 125.6° C., respectively. Fourthchambers, if they had been depicted, would have preferred temperatureranges of about 119.2 to 122.2° C., and about 116.4 to 119.2° C.,respectively.

Like adsorption chiller cycles, half of each stage is adsorbing for halfof the cycle and desorbing for the other half. However, unlike chillercycles, this cycle produces no heat pumping effect. To distinguish thetwo phases of operation, the term “forcing” is used when heat is inputto drive the two adsorbent beds in each stage out of equilibrium, and“relaxing” when the beds are allowed to return to equilibrium.

As shown in FIG. 1, during the “forcing” phase, the two beds in eachstage (21 and 22, 23 and 24, 28 and 29) are driven out of equilibrium.Note that while only three stages are shown here, many stages may beutilized. Preferably, the number of stages is 6 or more, and morepreferably the number of stages is between 12 and 40. In this phase, thebed operating at the highest temperature (21) is desorbed with heat(115) from a heat source (15), including but not limited to solar heat,while the hotter beds in every subsequent stage (23 and 28) is heated byrecycling the heat of adsorption transferred between it and the colderbed of the next higher temperature stage (22 and 24, respectively) viaboiled vapor, a circulating fluid, or any other method (see FIG. 3,elements 50, 55, and 30). During each phase, although heat istransferred between stages, water vapor produced from the input solutionfrom the evaporator of each stage is kept within that single stage.Water vapor desorbing from (21) will pass into a condenser (40) where itwill condense (120) and transfer its latent heat (121) to an evaporator(41). This process water will typically be passed back through a heatexchanger (70) to transfer its sensible heat to the incoming solution.The evaporator (41) will evaporate the input solution (122), and thatvapor will adsorb into the next bed (22). The heat of adsorption from(22) is then transferred (123) to desorb the next stage (23), and theprocess repeats through each stage until the final stage, which iscooled by seawater when used for seawater desalination (69) after itflows through a seawater intake (80), but can also be cooled by anyother external heat sink, which can include an evaporative coolingtower, or heat exchange with the ambient air, the input water source, orthe ground. A fraction of the cooling seawater will serve as intakewater (61), typically passed through one or more heat exchangers (70)with either or both the distillate and the brine before being fed to anevaporator (41), typically that of the first stage. Seawater not used asintake water (61) will typically be rejected (63). Brine from one stagewill be transferred (130) to next stage, passing through a heatexchanger (70) if the next stage operates at a lower brine temperature.No exit brine (60) heat recovery is necessary due to the low exit brinetemperature, which is preferably around 23.6° C. or below 40° C.

As shown in FIG. 2, when the system is in the “relaxing” phase, agreatly reduced amount or no external heat is needed for waterproduction. Each bed will produce the same amount of water peradsorb-desorb cycle. Therefore, the thermal GOR/PR of this device isideally equal to the number of beds rather than the number of stages, 32in a preferred design (16 stages, 2 beds per stage, also see FIG. 9).There will be losses from imperfect insulation and the heating ofcomponents, but these effects are only estimated to be around 5%. Thehot and cold beds of each stage are allowed to “relax” or equilibrate bycirculating fluid or by boiling and condensing vapor using a set ofinternal boiling tubes (see FIG. 4, element 55) embedded within theadsorbent matrix. As each hotter bed (21, 23, 28) cools, it will adsorb(122) vapor from brine, while each colder bed (22, 24, 29) will desorb(120) water into the condenser as it warms, transferring its latent heat(121) to the evaporator. The system is therefore in energy balance as anequal amount of water is adsorbing/desorbing and evaporating/condensing.This “relaxing” phase can be simply thought of as two adsorption bedsthat have been thermally driven apart equilibrating their temperaturesand uptake, the amount of adsorbed water.

The schematics shown in FIGS. 3 and 4 show schematics of connectedadsorption stages operating in the two modes, “Forced” (FIG. 3), and“Relaxation” (FIG. 4), while FIG. 7 shows a physical drawing of a singleadsorption stage with sectional cutouts on one bed to highlight internalstructures. Dashed lines (see, e.g., 30, 31, 32) indicate vapor, solidlines (see, e.g., 50, 62) indicate liquid water, long dashed lines (see,e.g., 61) indicates input water, while dotted lines (see, e.g., 60)indicate the residual brine stream, which flows serially through eachstage starting at the highest temperature. Each adsorption stage (e.g.,FIGS. 3, 4, and 7, elements 21/22, 23/24) is composed of two beds, a hotand a cold, with a flat plate (FIG. 7 element 44) evaporator (FIGS. 3,4, and 7, elements 41, 43) and condenser (FIGS. 3, 4, and 7 elements 40,42) between them which evaporates (32, 35) input water and condenses(31, 34) distillate. Two vapor valves (FIG. 7 elements 36, 37) connecteach bed to either the condenser or the evaporator, and each bed hasboth a grid of hollow tubes or channels (55), typically copper tubes,serving as a boiler/condenser to transfer heat via vapor (30) using aseparate vapor plenum (FIG. 7, element 38) through valved ports betweenbeds (FIG. 7, element 39) and a grid of vertical hollow channels (56) totransfer water vapor mass to/from the condenser/evaporator.

The heat transfer tubes and vapor plenum (FIGS. 3, 4, and 7, elements55, 38) are a closed-loop system transferring only heat betweenadsorption beds, and is separated from the condenser/evaporator. Whilethe heat flow in this closed-loop boiler is always from warmer tocolder, a liquid pump, a liquid valve, and a vapor valve allows eachadsorbent bed to transmit or receive heat in the form of vapor fromadjacent beds, as shown by the returning liquid condensate flows justbelow the beds (50). The hot bed of the first stage operates at thehighest temperature (21) is typically heated and desorbs water duringthe “forcing” phase (FIG. 9 element 324) using, for example, a thermalreservoir heated by solar energy (20), whereas the hot beds ofsubsequent stages (in the two stage example of FIGS. 3 and 4, element 23is the only other hot bed), the heat comes from the sensible heat andlatent heat of adsorption of the cold bed of the previous stage. Thecold bed of the last stage (in this case, 24) transmits its final wasteheat into a thermal sink (25), which is the input/cooling water sourcein this preferred design. Vapor (30) generated in the boiler tubes (55)by either the thermal reservoir (20) or the cold adsorption bed of eachstage (22, 24) passes to the hot bed of the next stage or the thermalsink for the final cold bed (21 for 20, 23 for 22, and 25 for 24), andbecause the destination is at a lower temperature than the origin, thevapor (30) condenses in the vertical copper tubes (55) and latent heatis transferred to the destination, with the condensed liquid waterpumped (50) back to fill the boiling tubes at the origin (20, 22, 24).The latent heat of vaporization is recycled from each condenser to theevaporator of the same stage to evaporate the input solution. The boilerheat transfer coefficient is governed by nucleate pool boiling andmethods of boiling surface enhancements exist to minimize the boilingsuperheat, preferably using wall surface modification such as powdersintering. FIG. 4 shows that during “Relaxation”, heat is no longerneeded from the thermal reservoir (20) nor exhausted to the heat sink(25), but is instead transferred from the hot beds (21,23), which is nowvalved to the evaporators (41, 43), adsorbing water vapor from theevaporator (32, 35), and boiling the water in the boiling tubes (55)filled by the liquid condensate pump (50), to the cold beds of the samestage (22, 24), which is now valved and desorbing water vapor (31,34) tothe condensers (40,42). The latent heat of vaporization from thecondensers is again recycled to evaporate the input solution in theevaporator.

One embodiment of a single adsorption bed (200) is shown in FIGS. 5-7.As shown in FIG. 5, the internal components of the bed (200) comprise avacuum-tight casing (210). The casing (210) is shown that has a surface(220) defining at least one recess (225) into which at least one portionof a condenser or evaporator can be positioned. The casing (210) alsocomprises a second surface (230) defining at least one second opening(235), each second opening (235) connecting to at least one of acondenser or evaporator. A vapor valve (236) is typically positionedwithin or around each second opening (235). Each second opening (235) iscapable of allowing vapor to exit after being desorbed or enter in orderto be adsorbed by adsorbent. The adsorbent is part of a highly parallelwinding defining the adsorbent bed coils (240). As shown in FIG. 6, theadsorbent bed coils (240) comprise powdered silica gel adsorbent (254)packed into tubing (250) with fins (252), kept in place with an outercovering (256), such as a mesh wrap. Significant quantities of silicagel are utilized for each bed. In preferred embodiments, between 100 and5000 kg of adsorbent are used for each bed. In more preferredembodiments, between 250 and 500 kg are used. In a most preferredembodiment, 450 kg of silica gel will be used for each bed.

FIG. 7 shows one embodiment of a single stage (11) using vapor totransfer and recycle the heat of adsorption between beds, a pair ofadsorption beds (21, 22) is combined with a flat plate (44)evaporator/condenser (40, 41) to make a single stage module. A flatplate evaporator/condenser is identical to the common flat plate heatexchanger, but with extra vapor and liquid inlet and outlets. The highsurface area evaporator/condenser minimizes thermal resistance betweenthe brine on the evaporator side and the distillate on condenser side,which keeps them isothermal. These flat plate evaporator/condensers arestandard industrial equipment already made from metals formulated toavoid corrosion and pitting in hot seawater.

In one embodiment shown in FIG. 8, adsorption beds are connected toeither the condenser or the evaporator alternately using valves (236),but no vapor adsorbed and desorbed flows from any adsorbent bed to anyother, only from/to the evaporator/condenser. Each stage is apressure/vacuum vessel evacuated of air to, for example, about 0.5-10kPa with either vacuum pumps or steam purges before the introduction ofbrine. Non-condensable gasses are kept out of the stages bypre-deaeration of the input seawater at an appropriately chosentemperature—for example, about 104-118° C. Each stage is thermallyinsulated on all sides, and heat transfer between beds is achieved usingwater circulation through the finned tubing. The electrical powerrequired for circulation pumps comprise most of the electricity needed.

By connecting adsorbent coils in parallel with a manifold, minimizingflow lengths, and using efficient pumps, the total electrical energyintensity for the cycle may be reduced to 0.5 kWh/m³ or less. This canbe generated with a bank of photovoltaic (PV) panels.

In one embodiment, the only surface to contact brine will be one side ofthe evaporator/condenser, which will not only be designed for automatedacid or chemical cleaning of scale build-up, but is also a maintainablecomponent than can be disassembled and pressure washed. This is incontrast to typical MED plants with extended heat transfer surfaceswhich can be difficult to maintain.

In one embodiment, manufacturing is relatively simple due to the lowcomponent count. Each distiller is assembled from a large number ofidentical adsorption stages. As shown in FIG. 8, each stage (300) ismade from two identical adsorption beds (200). The adsorption beds (200)will be made to serve as the frame for the flat plateevaporator/condenser (260) and the heat exchanger plates can bemanufactured from a variety of corrosion resistant metals. Theengineering and fabrication of each adsorption stage will need to becarefully designed, as each stage will need to be sealed into threesealed compartments (the area within each casing (210) and the volume ofspace between casing (210) and an upper surface (270) through whichvapor can pass through one of the vapor valves (236) and enter one ofthe hollow channels (280), comparable to channels (55) in FIGS. 1 and 2.The hollow channels (280) will need to be in place during the mechanicalcompression of the adsorbent material. Each stage also may containmultiple vapor valves; this figure utilizes six vapor valves (236).

For the purpose of illustrating how the cycle works, this disclosurerefers to a two stage system as shown in FIGS. 3-4 and assume that atthe start, both beds in Stage 1 for hot (21) and cold (22) are at thesame temperature, but at a higher temperature than beds in Stage 2 (23and 24), which are also at the same temperature.

In the ‘Forced” (i.e., heat-driven) phase, first bed (21) is heated bycondensing vapor in the boiler tubes generated by the heat source (20).Condensed water in the boiler tubes is pumped back to fill the boilertubes in the heat source, as indicated by the line (50) just below thebed and the heat source.

The first bed (21) is open to the condenser, and the increase intemperature causes the adsorbent to desorb vapor increasing the watervapor pressure and temperature in the condenser. This causescondensation and a transfer of the heat of vaporization to the inputwater in the evaporator. The second bed (22) is open to the evaporatorand its boiler tubes are full and transferring heat to the third bed viavapor (23). The cooling of the second bed (22) causes it to adsorb thevapor from the evaporator. The heat of adsorption from second bed (22)will continue to transfer to third bed (23) until it reaches theequilibrium uptake at the lower temperature.

Second stage beds (23 and 24) started at the same temperature, but withthird bed (23) receiving heat from second bed (22), it will also desorbvapor which will condense and evaporate more input water which will beadsorbed by fourth bed (24). The final bed exhausts adsorption heat to acondenser (25) cooled by the final residue water, distillate, andcooling water.

At the end of the “forcing” phase, the adsorption beds have becomeseparated in temperature and uptake, with each of the hot beds hotterand drier than the cold beds in each stage. The “relaxation” phaseconnects the hot and cold beds of each stage (i.e., 21/22, 23/24) toallow them to come back to equilibrium. As heat transfers from the hotto cold bed, the cold bed desorbs vapor into the condenser, which inturn evaporates input water adsorbed by the hot stage.

Adsorption Uptake Equilibrium and Kinetics

The adsorption distillation cycle depends on the “pull-push” action ofthe adsorbent to pull vapor when adsorbing and push when desorbing, so adiscussion of adsorbent equilibrium and kinetics is necessary.

The equilibrium uptake (kg/kg, adsorbed water mass per mass ofadsorbent) of the adsorbent has been measured and published by othersand can be fitted as a function of a single variable, the free energy ofadsorption, ΔF=−RT·ln(P_(water)/P_(saturated)), where P_(saturated) isthe saturated water vapor pressure at the temperature of the adsorbent.There were no significant differences between fits using Aristov'sformulas based on the Dubinin-Polanyi potential or fits using Tóth'sequation used by Chua. A computer program based on these equations waswritten to calculate cycle parameters. A plot of the equilibrium uptakeof the adsorbent is shown in FIG. 9.

In FIG. 9, the uptake is defined as the mass adsorbed per mass of theadsorbent (kg/kg) with numeric values (315) of each contour (310) shownnear the top. The saturate vapor pressure line (350) is also shown as adashed line. One embodiment of our cycle reaches 0.58 maximum uptake. Byoperating the condenser and evaporator isothermally, the parallelogramof a typical adsorption chiller cycle has been compressed into ahorizontal line. The thermodynamic theory of this cycle has beendiscussed by others, where all possible temperature combinations of asingle stage condenser/evaporator were analyzed theoretically.

To make the plot easier to read, the y-axis is plotted as thetemperature of the water with the saturated vapor pressure rather thanthe typical logarithmic vapor pressure scale. This is done because weare interested in the temperature of the water in the evaporator, whichdetermines the vapor pressure over the adsorbent.

The operating range of each adsorption bed (320) is shown overlaid onthe uptake contours (310) in FIG. 9. The horizontal black lines indicatethe operating temperature ranges for each stage. Arrows above(322)/below (324) the black line indicate the expected temperaturemovement of each hot and cold bed for each stage during the heatdriven/relaxation modes, respectively. Note the arrows do not fullyapproach adjacent beds for either mode of operation, this is merely aresult from the modelled thermal approach between beds for a cycle timeof 480 seconds.

In one embodiment, the uptake change for each bed is designed for 3.2%.With 325 kg in each bed, 32 total beds, and a cycle time of 480 s, thedaily output from one distiller is 60 m³. The design includes sufficientsolar collection and hot water storage with a swing from 150-180° C. topower the distiller during nighttime.

Adsorption kinetics determines the water production rate. One embodimentof a cycle has been designed using the linear driving force kineticequation found in a number of published journal articles fromindependent research groups. In this embodiment, each stage is designedfor an uptake swing of 0.034 kg/kg within 6 minutes. Adsorption beds atlower temperatures in FIG. 9 have slower kinetics, and are thereforedesigned with larger initial uptake differentials.

One embodiment of this system uses a serial flow pattern to achieve highrecovery ratios (80%), where most of the water is extracted from thebrine, rather than discharged. This can be increased further forwastewater remediation applications where minimal residual discharge isdesirable.

Top Brine Temperature for seawater desalination

One example design, similar to that illustrated in FIG. 1, is based on150° C. input heat, since this the temperature of the highest stage.This is useful to illustrate how the adsorption distiller is also ableto use higher temperature or exergy heat sources to increase efficiencywithout being limited by the scaling of inverse temperature solubilitysalts including CaSO₄, Mg(OH)₂, and Ca₃(PO₄)₂. The scaling from thesesalts on heat transfer surfaces typically limits the highest temperatureseawater can be heated to between 90° C. to 110° C., depending on theanti-scaling chemicals used and whether the input water is pretreatedusing micro/nano-filtration to partially remove salt ions. Theadsorption distiller overcomes the top brine limit because the adsorbentis operated above the brine temperatures allowing higher temperatureinput heat to drive more stages even while the seawater remains ataround 100° C. Also, because the evaporator is operated as a pool boilerrather than a film evaporator, local scaling, where local concentrationlimits are violated due to excessive local evaporation, should havereduced severity. We currently do not anticipate using anti-scalantchemicals, relying instead on techniques such as acid washing orTaprogge cleaning balls in the evaporator.

The top brine temperature in one embodiment was selected to remain below120° C. to stay below the solubility limit of hard CaSO₄ scaling. Thebrine flow is serial from one evaporator to the next in decreasingtemperature order and increasing salinity, as seen in FIGS. 1-2. Inthermal desalination without nanofiltering, the top brine temperaturelimits thermal efficiency because the brine is the hottest component inthe system. However, in the adsorption cycle, the adsorbent can beoperated at a higher temperature than the brine to achieve higherthermal efficiency. Silica gel used in desiccant wheels is reliablyregenerated for many cycles above 150° C.

FIG. 9 illustrates the continuity of the cycle. Each adsorption stagefollows the next in temperature, but the input water experiences severaltemperature variations as it flows serially from the highest temperaturestage to the lowest. In FIG. 9, stages 1-4 operate at theevaporator/condenser temperature of 104° C., stages 5-7 at 82° C.,stages 8-10 at 61° C., stages 11-13 at 41° C., and stages 14-16 at 20°C. Other temperatures are also envisioned, and will preferably rangefrom around 20° C. to around 120° C. Both the residue water and thedistillate will be at these evaporator/condenser temperatures, and areheat exchanged with the incoming input water using flat plate heatexchangers (70) as shown in FIGS. 1-4.

FIG. 9 also shows the trajectory of each bed during both phases of thecycle. Each horizontal black segment in FIG. 9 represents the operatingrange of both beds in an stage. It can be seen that the stages in thisembodiment operate at brine temperatures between about 20° C. to about105° C., and that the adsorbent bed temperatures decrease monotonicallythrough the stage chain from about 150.0° C. to about 30° C. In someembodiments, multiple stages—for example, three to four stages—mayoperate at each brine temperature in a sequence of decreasing brinetemperatures. In these embodiments, intake seawater to product waterheat exchangers may be introduced after every three to four stages.Arrows above segments show beds “relaxing” towards the midpoint intemperature and uptake, with arrows pointing left denoting cooling andarrows pointing right denoting heating. The opposite occurs during the“forcing” phase, where beds in each stage are driven out of equilibriumby the heat source (FIGS. 3 and 4 element 20), cooling by the heat sink(FIGS. 3 and 4 element 25), or by thermal connection to a bed in anadjacent stage (FIGS. 3 and 4 elements 22, 23).

System Integration and Packaging

FIG. 10 shows a view of one embodiment of a single desalination unit(400), packaged into two housing units (410, 450), which in thisembodiment can be, for example, shipping containers. The low-profilesolar thermal collector arrays (470), PV panels (460), hot water/thermalstorage (420) and batteries (480) make up most of the system. In someembodiments, the thermal storage (420) is configured to provide about750 kWh, and the batteries (480) are configured to provide about 7 kWh.The adsorption modules (440) and hot water storage (420) may beinsulated by, for example, about 30 cm of rigidpolyurethane/polystyrene, which is not shown in the figure for clarity.Also not shown are plumbing connections, valves, circulation pumps, andcontrol systems.

The thermal collector arrays and PV arrays may be deployed outside thehousing units, while the adsorption modules operate inside the housingunits. In some embodiments, the housing units may be behind PV arrays,and may act as support structures for the PV arrays. These housing unitsmay be located in practically any location, including near water sourcessuch as oceans or seas.

Table 1 summarizes specifications for one embodiment of the desalinationunit.

TABLE 1 Water production (annual average) 60 m³/day or 16000 gpd ThermalGOR/PR ~28 Energy intensity 23 kWh_(th) + 0.1 kWh_(e) / m³ Adsorptioncycle time 480 s Adsorbent mass per bed 325 kg Recovery Ratio 72% Landfootprint 400 m² or 4306 ft² Solar thermal collection area 450 m² (incl.night storage) PV panel installed watts 2250 W (incl. night storage)Assumed insolation 5 kWh/m²/day

Other Features

Exergy efficiency on the adsorption distiller can be optimized in realtime to maximize water production based on changing conditions.Optimization may be based on only three (3) input parameters: input heattemperature, exhaust temperature, and the amount of heat available. Theonly actuators for control may be the switching times and durations ofthe relaxation and heat driven modes. Automation of the process canoccur using a real-time optimized controller using a low-power embeddedcomputer with cellular connectivity, such as the Raspberry Pi and/orParticle Electron, to allow remote control and data logging ofoperational units worldwide.

What is claimed is:
 1. A method of distilling water, comprising thesteps of: a. providing a plurality of stages, each stage comprising ahot adsorbent bed and a cold adsorbent bed, and each stage has an upperand lower operating temperature limit, the difference between the upperand lower operating temperature limit being less than about 20° C.; b.beginning a forcing phase, wherein the forcing phase comprises the stepsof: i. providing an external heat source to heat the hot bed of a firststage to a first temperature; ii. desorbing water vapor from the hot bedof the first stage and flowing water vapor into a first condenser; iii.condensing water vapor in the first condenser to form a liquid water andremoving at least some of the liquid water from the first condenser; iv.providing a solution comprising water and at least one dissolvedimpurity to a first evaporator, the solution having a temperaturepredetermined to suit the equilibrium uptake of an adsorbent, where asuitable temperature is predetermined by first selecting both a desiredoperational temperature range and uptake range for the adsorbent, thenselecting the solution temperature such that the saturated water vaporpartial pressure corresponds to the desired adsorbent temperature anduptake range; v. transferring a forcing phase latent heat ofvaporization from vapor condensing in the first condenser to the firstevaporator to evaporate the solution comprising water and at least onedissolved impurity to form water vapor; vi. adsorbing water vapor fromthe first evaporator into the cold bed of the first stage; vii.transferring the heat of adsorption generated by the cold bed of thefirst stage to heat a hot bed of a second stage to a second temperatureless than the first temperature using vapor generated through theconduction of heat from the cold bed of the first stage into at leastone sealed tube and at least one sealed manifold chamber connecting thetwo beds, wherein the sealed tubes and chambers are evacuated ofnon-condensable gases and partially filled with a volatile liquid; viii.repeating steps ii-vii for each of the plurality of stages until each ofthe beds has had water vapor desorbed from the bed or adsorbed into thebed; ix. exhausting the heat of adsorption generated by the cold bed ofthe final stage externally; and c. ending the forcing phase andbeginning a relaxing phase, wherein the relaxing phase comprises thesteps of: x. transferring both the sensible heat of the adsorbent bedand the heat of adsorption from the hot bed of the first stage to thecold bed of the first stage using vapor generated through the conductionof heat from the cold bed of the first stage into at least one sealedtube and at least one sealed manifold chamber connecting the two beds,wherein the sealed tubes and chambers are evacuated of non-condensablegases and partially filled with a volatile liquid; xi. desorbing watervapor from the cold bed of the first stage into the first condenser;xii. condensing water vapor in the first condenser to form a liquidwater and removing at least some of the liquid water from the condenser;xiii. providing the solution comprising water and at least one dissolvedimpurity to the first evaporator; xiv. transferring a relaxing phaselatent heat of vaporization from vapor condensing in the first condenserto the first evaporator to evaporate the solution comprising water andat least one dissolved impurity to form water vapor; xv. adsorbing watervapor from the first evaporator into the hot bed of the first stage,generating a heat of adsorption; xvi. repeating steps x-xiii for each ofthe plurality of stages; and d. ending the relaxing phase, wherein thesolution comprising water and at least one dissolved impurity is used toremove heat from at least one bed of at least one stage, wherein atleast a portion of the solution enters the first evaporator, and whereinat least a portion of the solution is transferred from the evaporator ofeach stage to the evaporator of one or more subsequent stages.
 2. Themethod according to claim 1, wherein the plurality of stages areconfigured to use a serial flow pattern to achieve a water recoveryratio of at least 80%.
 3. The method of claim 1, wherein the solutioncomprising water and at least one dissolved impurity has been heatedprior to being provided to the first evaporator by extracting sensibleheat from the solution exiting at least one evaporator, the condensedliquid water exiting from at least one condenser, or both.
 4. The methodof claim 1, wherein the first stage operates at temperatures between 60°C. and 210° C., and providing the plurality of stages comprisesproviding at least three stages.
 5. The method of claim 1, furthercomprising providing a plurality of solar cells to provide electricalpower, and a plurality of solar thermal collectors to provide thermalpower.